(1) Field of the Invention
The present invention relates to semiconductor memory devices and more particularly to a method of forming a split-gate flash memory cell having a self-aligned source as well as a floating gate self-aligned to a control gate.
(2) Description of the Related Art
A split-gate flash memory device is characterized by its split-gate side (between the control gate and the drain) and the stacked-side (between the floating gate and the source). Conventionally, the floating gate is not self-aligned to the control gate, and neither is the self-aligned source commonly used. Without self-aligned source, the problem of punch-through from the source to the control gate is encountered which in turn causes programming fails. And in the absence of alignment between the floating gate and the control gate, the misalignment causes variability of the coupling ratio, which also causes weak programmability in the cell. As is known, the coupling ratio affects the program speed, that is, the larger the coupling ratio, the faster is the programming speed, and is not a fixed value by virtue of the variability of the channel length and hence that of the overlap between the floating gate and the source. Usually, if channel length is increased through greater lateral diffusion in the source region, punch-through occurs due to excessive current well below the threshold voltage. It is disclosed later in the embodiments of this invention that these problems can be alleviated by forming self-aligned source and also self-aligned floating gate with respect to the control gate. As an added advantage, it is shown that the cell size can be reduced by using self-alignment methods and self-aligned structures of this invention.
Over the years, numerous improvements in the performance as well as in the size of memory devices have been made by varying the simple, basic one-transistor memory cell, which contains one transistor and one capacitor. The variations consist of different methods of forming capacitors, with single, double or triple layers of polysilicon, and different materials for the word and bit lines. In general, memory devices include electrically erasable and electrically programmable read-only memories (EEPROMs) of flash electrically erasable and electrically programmable read-only memories (flash EEPROMs). Many types of memory cells for EEPROMs or flash EEPROMs may have source and drains regions that are aligned to a floating gate or aligned to spacers. When the source and drain regions are aligned to the floating gate, a gate electrode for a select transistor is separate from the control gate electrode of the floating gate transistor. Separate select and control gates increase the size of the memory cell. If the source and drain regions are aligned to a spacer formed after the floating gate is formed, the floating gate typically does not overlie portions of the source and drain regions.
Programming and erasing performance is degraded by the offset between the floating gate and source and drain regions.
Most conventional flash-EEPROM cells use a double-polysilicon (poly) structure of which the well known split-gate cell is shown in FIG. 1a. Here, two MOS transistors share a source (25). Each transistor is formed on a semiconductor substrate (10) having a first doped region (20), a second doped region (25), a channel region (23), a gate oxide (30), a floating gate (40), intergate dielectric layer (50) and control gate (60). Substrate (10) and channel region (23) have a first conductivity type, and the first (20) and second (25) doped regions have a second conductivity type that is opposite the first conductivity type.
As seen in FIG. 1a, the first doped region, (20), lies within the substrate. The second doped region, (25), also lies within substrate (10) and is spaced apart form the first doped region (20). Channel region (23) lies within substrate (10) and between first (20) and second (25) doped regions. Gate oxide layer (30) overlies substrate (10). Floating gate (40), to which there is no direct electrical connection, and which overlies substrate (10), is separated from substrate (10) by a thin layer of gate oxide (30) while control gate (60), to which there is direct electrical connection, is generally positioned over the floating gate with intergate oxide (50) therebetween.
In the structure shown in FIG. 1a, control gate (60) overlaps the channel region, (23) under the floating gate, (40). This structure is needed because when the cell is erased, it leaves a positive charge on the floating gate.
As a result, the channel under the floating gate becomes inverted. The series MOS transistor (formed by the control gate over the channel region) is needed in order to prevent current flow from control gate to floating gate. The length of the transistor, that is the overlap of the control gate over the channel region (23) determines the cell performance. Furthermore, edges (41), (43) can affect the programming of the cell by the source size and hot electron injection through the intergate dielectric layer (50) at such edges. Hot electron injection is further affected by, what is called, gate bird's beak (43) that is formed in conventional cells. On the other hand, it will be known to those skilled in the art that corners such as (41) can affect the source coupling ratio also.
To program the transistor shown in FIG. 1a, charge is transferred from substrate (10) through gate oxide (30) and is stored on floating gate (40) of the transistor. The amount of charge is set to one of two levels to indicate whether the cell has been programmed "on" of "off." "Reading" of the cell's state is accomplished by applying appropriate voltages to the cell source (25) and drain (20), and to control gate (60), and then sensing the amount of charge on floating gate (40). To erase the contents of the cell, the programming process is reversed, namely, charges are removed from the floating gate by transferring them back to the substrate through the gate oxide.
This programming and erasing of an EEPROM is accomplished electrically and in-circuit by using Fowler-Nordheim tunneling as is well known in prior art. Basically, a sufficiently high voltage is applied to the control gate and drain while the source is grounded to create a flow of electrons in the channel region in the substrate. Some of these electrons gain enough energy to transfer from the substrate to the floating gate through the thin gate oxide layer by means of Fowler-Nordheim tunneling. The tunneling is achieved by raising the voltage level on the control gate to a sufficiently high value of about 12 volts. As the electronic charge builds up on the floating gate, the electric field is reduced, which reduces the electron flow. When, finally, the high voltage is removed, the floating gate remains charged to a value larger than the threshold voltage of a logic high that would turn it on. Thus, even when a logic high is applied to the control gate, the EEPROM remains off. Since tunneling process is reversible, the floating gate can be erased by grounding the control gate and raising the drain voltage, thereby causing the stored charge on the floating gate to flow back to the substrate.
In the conventional memory cell shown in FIG. 1a, word lines (not shown) are connected to control gate (60) of the MOS transistor, while the length of the MOS transistor itself is defined by the source (25) drain (20) n+ regions shown in the same Figure. As is well known by those skilled in the art, the transistor channel is defined by masking the n+ regions. However, the channel length of the transistor varies depending upon the alignment of the floating gate (40) with the source and drain regions. This introduces significant variations in cell performance from die to die and from wafer to wafer. Furthermore, the uncertainty in the final position of the n+ regions causes variations in the series resistance of the bit lines connected to those regions, and hence additional variation in the cell performance.
In prior art, Hsia, et al., of U.S. Pat. No. 4,861,730 teach a process for producing a high density split gate nonvolatile memory cell which includes a floating gate and a control gate that is formed above the floating gate. The drain region is self-aligned to the floating gate and source region is self-aligned to the control gate. In U.S. Pat. No. 5,063,172, Manley discloses an integrated circuit fabrication method that utilizes a conductive spacer to define the gate length of the series select transistor in a split-gate memory cell, thereby eliminating misalignment problems.
On the other hand, Chen, et al., in U.S. Pat. No. 5,824,484 disclose a sidewall select gate which is formed in conjunction with a semiconductor doped oxide to form a nonvolatile memory cell. In another U.S. Pat. No. 5,750,427 Kaya, et al., disclose a non-volatile split-gate memory cell which can be programmed with only a five volt power supply and is fabricated using standard transistor processing methods. Also, a method to improve erase speed of split-gate flash is taught by Hsieh, et al., in U.S. Pat. No. 5,858,840 by judiciously implanting nitrogen ions in the first polysilicon layer and then removing them from the area where the floating gate is to be formed.
While prior art offers different approaches for forming different split-gate flash memory cells, the present invention discloses a still different method where the floating gate is aligned to the control gate. In the split-gate cells shown in FIG. 1 of prior art, floating gate (40) is not aligned to control gate (60) for the dimensions (x) and (y) are different. This is also shown in a top view given in FIG. 1b. It is disclosed later in the embodiments of this invention, where this variability is avoided and the performance of the split-gate flash memory cell is improved.